Spann Nicole, Harper Elizabeth M, Aldridge David C
Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK.
Naturwissenschaften. 2010 Aug;97(8):743-51. doi: 10.1007/s00114-010-0692-9. Epub 2010 Jun 22.
Asian clams (Corbicula fluminea) with abnormally thickened shell valves were found in four rivers in the UK (Rivers Yare, Waveney, Thames and New Bedford River). The material making up these malformations was the rare calcium carbonate polymorph vaterite. Vaterite is seldom found in the natural environment because it is less stable than the other calcium carbonate polymorphs (aragonite and calcite). In the few reported cases of vaterite formation in molluscs, it is usually related to unusual biomineralisation events such as shell regeneration, pearls and initial stages of shell formation. We compared two populations from the Rivers Yare and Waveney in the Norfolk Broads, UK, one (River Waveney) displaying dominantly the normal Corbicula shell form with aragonitic shells. In the River Yare population, all individuals sampled had shell deformations to different extents. These deformations were apparent as bulges on the inside of the ventral shell margin. X-ray diffraction confirmed that the shell material in the bulges of recently collected clams was vaterite. Other parts of the deformed shells were aragonitic. The shell deformations alter the shell morphology, leading to higher and wider shells. The shell microstructure is fibrous in the vateritic parts and crossed-lamellar in the aragonitic parts of deformed or non-deformed shells. The cause for the malformations is probably a disrupted biomineralisation process in the bivalves. Fossil Corbicula specimens from the late Pleistocene had similar deformations, suggesting that this is not a response to anthropogenic causes, such as pollution.
在英国的四条河流（耶尔河、韦弗尼河、泰晤士河和新贝德福德河）中发现了壳瓣异常增厚的亚洲蛤蜊（河蚬）。构成这些畸形的物质是罕见的碳酸钙多晶型球霰石。球霰石在自然环境中很少见，因为它比其他碳酸钙多晶型物（文石和方解石）更不稳定。在少数已报道的软体动物中球霰石形成的案例中，它通常与异常的生物矿化事件有关，如贝壳再生、珍珠形成以及贝壳形成的初始阶段。我们比较了英国诺福克湖区耶尔河和韦弗尼河的两个蛤蜊种群，其中一个种群（韦弗尼河）主要呈现具有文石质贝壳的正常河蚬壳形态。在耶尔河的种群中，所有采样个体都有不同程度的壳变形。这些变形表现为腹侧壳边缘内侧的凸起。X射线衍射证实，最近采集的蛤蜊凸起部位的壳物质是球霰石。变形壳的其他部分是文石质的。壳变形改变了壳的形态，导致壳更高更宽。在变形或未变形壳的球霰石部分，壳微结构是纤维状的，而在文石部分是交错层状的。畸形的原因可能是双壳类动物生物矿化过程受到干扰。来自晚更新世的河蚬化石标本也有类似的变形，这表明这不是对污染等人为原因的反应。
